HSBC's Swiss banking arm helped some of their richest clients conceal their assets and avoid taxes, according to a huge trove of leaked accounts released by an international collaboration of news outlets.
The global bank, which is headquartered in the city of London and has offices in 74 counties and territories across six continents, has faced criminal charges over the documents in several countries.
Holding a secret bank account is not illegal in Swiss law, but using them to deliberately conceal assets in order to dodge tax is.
The documents show how the bank repeatedly reassured clients that it would not disclose details of accounts to national authorities, even if evidence suggested that the accounts were undeclared to national tax authorities, the ICIJ said.
The records are also said to show evidence of bank employees discussing a wide range of measures that would ultimately allow clients to avoid paying taxes in their home countries.
Almost 60,000 leaked files filed that provide details on the names, professions and value assets of more than HSBC clients.
The files were leaked by a former IT worker in the HSBC Private Bank (Suisse), Hervé Falciani, who downloaded the vast amounts of information in 2008 and then attempted to sell the files to banks in Lebanon. He was arrested by Swiss police on suspicion of stealing data.
After being questioned, he fled to France, and gave the data to the French authorities, who then indicted the London-based bank for money laundering and facilitating tax-fraud among other things.
The French paper Le Monde obtained the information, and shared it with the International Consortium of Investigative Journalists, who led this international investigation.
The data includes the details of 30,000 accounts holding almost £78 billion in assets. Among those implicated include politicians, sport stars, celebrities as well as criminals and traffickers.
When initially contacted by the ICIJ and its media partners, HSBC first insisted that ICIJ destroy the data, the consortium said.
HMRC, which received the data in 2010, has clawed back £135 million from some of the 3,6000 Britons identified as having potentially used the scheme, but it has been criticised as only one person has been prosecuted as a tax evader.
In a statement, HSBC said it had "implemented numerous initiatives designed to prevent it services being used to evade taxes or launder money" and stressed that standards had "significantly improved" since the time in question.
Although there are numerous legitimate reasons to have a Swiss bank account, in some cases individuals took advantage of bank secrecy to hold undeclared accounts.
HSBC's full response is available here.